Typical examples of heat-responsive polymer materials having ester bonds or acid amide bonds include partially oxidized polyvinyl alcohol and N-isopropyl acrylamides. It is known that the cloud point of an ester bond-type polymer or an alkylamide polymer would be gradually lowered with an increase in the carbon atom number in a side chain. It is therefore impossible to synthesize a heat-responsive polymer of the alkylamide type having a side chain with a large carbon atom number. In the case of an ester-bond type polymer or an alkylamide polymer, it is also difficult to provide a sufficient polarity in separating any protein.
N-Alkylacrylamides typified by N-isopropylacrylamide, which in polymer form are known to be temperature-responsive, have been frequently applied to DDS (Drug Delivery System) and separating agents. However, alkylacrylamide monomers showing temperature-responsiveness in polymer form carry exclusively alkyl groups with a small number of carbon atoms. Owing to this characteristic, these alkylacrylamides are poor in the hydrophobic nature or the hydrogen bonding properties, which makes it difficult to efficiently separate, adsorb and release all biological components or organic matters by using the these kinds of polymers. Although hydrogen-bonding groups can be introduced into these compounds by forming a copolymer with the use of a monomer having a hydrogen-bonding group, there arises a problem in this case, i.e., an increase in the cloud point or the disappearance of the temperature-responsiveness. As a result, a target substance, in particular, a biological component should be separated, adsorbed and released under severe conditions.
In polymer compounds showing structural changes due to external stimuli (temperature, pH, light, etc.), the structural changes result in changes in the characteristics of the polymers, for example, volume or hydrophilic/hydrophobic nature. For example, it is well known that poly(N-isopropyl acrylamide) shows a structural change in an aqueous solution depending on temperature. Namely, this compound is soluble in water in a low temperature side of 32° C. or below but becomes insoluble in water in a high temperature side exceeding 32° C. That is to say, it is a temperature-responsive polymer compound having a lower critical solution temperature (LCST). It is considered that such a polymer compound would show a hydrophilic nature and be dissolved in water in a swollen state in the low temperature side and, in the high temperature side, it would show a hydrophobic nature and be aggregated in a contracted state. By using these temperature-depending changes, temperature-responsive polymer compounds have been applied to drug delivery systems and high-functional materials such as separators.
To apply these temperature-responsive polymer compounds to high-functional materials, it is needed to use not only temperature-responsive polymer compounds having LCST but also those having the upper limit critical solution temperature (UCST), i.e., being insoluble in water in a low temperature side but becoming soluble in water in a high temperature side. When a protein unstable to heat is to be separated by adsorbing on a temperature-responsive polymer compound via a hydrophobic action, a temperature-responsive polymer compound having the UCST, i.e., showing a hydrophobic nature at low temperatures, is seemingly useful. At present, however, there are known temperature-responsive polymer compounds of few types having the UCST and it is difficult to newly develop temperature-responsive polymer compounds usable as efficient high-functional materials.
To obtain high-functional temperature-responsive polymer compounds, it is necessary to develop temperature-responsive polymer compounds having novel characteristics which are not achieved by the existing ones. In general, it is considered that a temperature-responsiveness is expressed owing to the balance between a hydrophilic moiety and a hydrophobic moiety. For example, a temperature-responsive polymer compound becomes insoluble in water with an increase in the carbon atom number in the side chain thereof. It is, therefore, difficult to synthesize a temperature-responsive polymer compound having a side chain with a large carbon atom number.